Sedimentation is a leading cause of surface water impairment. Sediment transported from rural and urban surfaces has the potential to change stream flow patterns, to transport other pollutants such as metals and hydrocarbons, and to degrade aquatic habitat. According to the National Water Quality Inventory: 2004 Report to Congress, prepared under sections 305(b) and 303(d) of the Clean Water Act, at least 9% of the stream miles and 7% of the lake acres assessed were found to be impaired by sediment and turbidity. To decrease the negative impacts of stormwater on surface waters, the United States Environmental Protection Agency (EPA) has developed a manual of Best Management Practices (BMP's), which recommends structures such as sediment and detention basins. Detention basins have outlets designed to temporarily detain stormwater runoff in order to attenuate peak discharges, while a primary purpose of sediment basins is to detain stormwater runoff long enough to allow sediments to settle. In both cases, however, there is a water quality benefit to be served by discharging cleaner water and retaining contaminants in the basin for stabilization and/or removal.
Sediment and/or detention ponds or basins are currently required for most new land disturbing activities (i.e., construction, roads, mining, forestry, and development). Generally speaking, more traditional agricultural entities have not yet been required to meet the same water quality standards of other industries, but that may change in the near future. One needs look no further than the increased environmental regulation of the hog, poultry, and dairy industries to see a general trend toward tightening environmental regulation, which could ultimately result in required sediment BMP's for disturbed agricultural lands.
Currently most communities regulate erosion and transport of sediment through a variety of required BMP's. These practices instruct a user how to reduce the sediment delivered to a creek, stream, pond, river, lake, or bay, but do not ensure that the user has implemented the BMP's well such that they are performing adequately. The U.S. Environmental Protection Agency (EPA) has written new regulations that the effluent from disturbed sites must have a turbidity of less than 280 Nephelometric Turbidity Units (NTU's). Although this rule is not yet implemented and is currently being appealed, it is clear that the United States is moving away from prescriptive BMP approaches towards performance-based approaches. This will drive users towards placing their money and efforts on erosion and sediment control technologies that meet performance-based turbidity requirements.
Detention basins are often used for reducing stormwater peak discharges, but there is regulatory movement towards demanding removal of contaminants from discharge even if there is little sediment being contributed from the associated drainage basin. Whether for sediment basins during soil disturbance or for post-disturbance detention basins, if pollutant removal is an important consideration, the common approach is to have a traditional vertical perforated riser as an outlet. This may not, however, be the most appropriate choice for pollutant removal, since such risers typically only force settling of 40%-70% of total suspended solids. Floating skimmers are available, and these function to discharge the cleanest water from the top of a basin, draining water by means of a single orifice submerged just below the surface such that only the cleanest water is discharged. Despite their benefits, floating skimmers are articulated by some means to maintain the orifice at just beneath the water level, and this required motion may be a potential source for failure. These skimmers are also prone to becoming stuck when the water is drained and the skimmer is lying on a drying muddy surface. Also, floating skimmers may be more susceptible to vandalism than are fixed traditional risers, as a floating skimmer may lie at the bottom of a basin when the basin is dry and be an attractive target for vandalism. Floating skimmers are also limited by a fixed outflow rate regardless of the stage (elevation of the water surface) in the basin, which limits their utility as detention basin outlets.
Accordingly, improvement is desired in the field of sediment basin outlet design towards improving stormwater discharge quality.
Now a brief history of the development of sediment and/or detention basin technology will be described. A traditional perforated riser for a stormwater basin is recommended by the EPA as of 2006. However, a riser was first used to discharge runoff from agricultural terraces in the 1940's by the United States Department of Agriculture (USDA) Soil Conservation Service (SCS) in Iowa. These risers were mounted flush to the ground and drained to a conduit below grade. The concept of using above-grade perforated risers to attenuate peak outflows by temporarily storing runoff above grade was first utilized in the 1960's by USDA SCS engineers in Iowa. Using a perforated riser outlet was found to decrease the peak flow rate from a terrace such that smaller subsurface piping could be used as a water outlet. The smaller flow rates and associated smaller pipes were found to be more cost effective than earlier systems that had very little storage and subsequently required larger pipes to carry large peak discharges during peak runoff periods. Riser intakes were later used to drain beef feedlots, and orifices of 1.59 cm (⅝ in) diameter or greater provided good flow control and were found to not readily clog with suspended solids or floating debris.
U.S. Pat. No. 5,820,751 issued Oct. 13, 1998 to Faircloth, Jr. describes a floating skimmer connected to a sediment basin outlet by a flexible outlet pipe. A single orifice is held at a shallow fixed depth below the water surface in a sediment basin. The orifice is subjected to a constant head, yielding a constant flow rate from the skimmer regardless of the stage within the basin. Although having a constant outflow simplifies the routing calculations defining the rate at which the runoff moves through the basin, basins utilizing a skimmer outlet must be made relatively large to detain larger storm events because they will generally drain much slower than they fill. Such a floating skimmer has several rotating parts that can be damaged by suspended or deposited sediments. Considering the muddy, gritty environment in which these devices operate, many components comprising the floating skimmer must work in unison for the skimmer to operate correctly. For example, the orifice assembly must be free to rotate as the water level increases or the discharge rate will be incorrect. Pivoting debris guards must be free to move or the single orifice could become clogged. Finally, the floating skimmer assembly must be free to raise and lower on the flexible hose attached at its base. If the skimmer becomes lodged in the muddy bottom of the basin, a large discharge rate of untreated water from the bottom of the basin would result. U.S. Pat. No. 4,015,629 issued Apr. 5, 1977 to Morgan et al. discloses an earlier adjustable flow floating weir assembly that may vertically adjust relative to liquid level in a basin. U.S. Pat. No. 4,517,091 issued May 14, 1985 to Yamanaka et al. provides for a swirling flow of liquid through a solids-liquid separator. More recently, U.S. Pat. No. 7,025,888, issued Apr. 11, 2006 to Thompson et al. provides a floating decanter for a basin which may prevent a drawing of floating solids or scum into a discharge outlet, functioning almost to the opposite effect of a skimmer.
Therefore, if an effective yet simple means were developed to increase the efficiency and ease of sediment retention within basins, this would be a valuable contribution and result in an improved skimmer system that can be applied to drain a sediment or detention basin (hereinafter, a sediment/detention basin). Such a skimmer system oftentimes is asked to serve multiple purposes simultaneously, namely reducing peak discharge in a controlled manner so as to reduce total suspended solids (TSS) or turbidity of the discharge. In addition, it would be advantageous if a skimmer system could: eliminate the need for moving parts, be less prone to failure or vandalism, be inexpensive to construct and install, have reduced maintenance, could accommodate variable discharge rates if desired, and if it contained its own secondary spillway, provide a water exit if the primary openings should clog.